The present invention relates to semiconductor processing, and in particular to the fabrication of an integrated circuits having both a vertical dynamic random access memory (DRAM) and other support circuits, and including an isolation trench that reduces corner device effects in the DRAM.
Vertical dynamic random access memory (DRAM) circuits have been employed to reduce effective cell size and increase memory density. Typically, a vertical DRAM memory cell comprises a vertical metal oxide semiconductor field effect transistor (MOSFET) connected to a trench capacitor below the MOSFET. In the vertical MOSFET, a surface of the channel region of the transistor is generally perpendicular to the primary surface of the substrate.
DRAM circuits may be made by separating the process steps for the vertical arrays and planar support devices, e.g. logic circuits. However, significant challenges exist in the manufacture of combined DRAM circuits where the fabrication of embedded vertical DRAMs is integrated into the process flow of the fabrication of planar support devices. Among these problems is the increased thermal budget, i.e., the cumulative effect of time at elevated temperature to which the implants for the vertical device are exposed. Thermal processing is typically involved in growing oxide layers such as the DRAM gate sidewall layers and the planar gate oxide in the support area, and may reach temperatures of 1000–1200° C. When implants for the vertical device are made before such extensive thermal processing, segregation often results. For example, shallow trench isolation (STI) with an oxide liner may be employed to isolate the DRAMs. In such case, the corner device of the transistor portion of the DRAM may be affected by the STI liner oxidation because of boron segregation into the liner, and resulting depletion in the channel adjacent the gate conductor, as a result of exposure to higher temperatures.
Accordingly, there is a need for a structure which reduces unwanted corner device effect of a vertical DRAM, particularly from boron depletion in areas adjacent the gate and STI.